A 29th-round draft pick by the Boston Red Sox in 2000, Gabbard started the 2006 season with Boston's Double-A affiliate Portland and posted a 9-2 record with a 2.57 ERA in 13 starts. Promoted to Triple-A Pawtucket on June 23, he went 1-3 with a 4.97 ERA in five starts. He earned a promotion to the Boston Red Sox on July 21 to replace Tim Wakefield, who was placed on the disabled list. In his major league debut on July 22, he allowed two earned runs in 52⁄3 innings to the Seattle Mariners and was charged with a loss. Gabbard's first major league win was on September 5 against the Chicago White Sox. Gabbard pitched seven shutout innings, surrendering three hits and striking out six in a 1-0 Red Sox win.

Gabbard was called up from Triple-A Pawtucket to pitch on May 20, 2007, against Atlanta in place of Josh Beckett. He had a good performance allowing 2 runs on 6 hits over 5 innings and got the win. He was then immediately optioned back to Pawtucket after the game. Gabbard started on June 26, 2007, against Seattle in place of Curt Schilling, who was placed on the disabled list. Gabbard had a sub-par performance, earning a no decision, and lasted only 31⁄3innings allowing 4 runs on 6 hits while walking six and striking out two. Gabbard performed well in his third start of the season on July 2, 2007, allowing three runs on three hits while walking four and striking out five, earning the win.

Gabbard was traded to the Texas Rangers on July 31, 2007, for relief pitcherEric Gagné, the same day the Atlanta Braves traded his high school teammate Saltalamacchia to Texas.

Gabbard made his Ranger debut on August 2, 2007, against the Cleveland Indians. He was pinned for the loss as the Indians won the game 5-0. Less than three weeks later, on August 22, Gabbard was the winning pitcher in a game where the Rangers outscored the Baltimore Orioles 30-3, the most runs scored in a game in the modern era.[3]

On May 8, 2008, in a game against the Seattle Mariners, Gabbard was charged on the mound by Seattle's Richie Sexson. Sexson threw his batting helmet at Gabbard after Gabbard's head-high pitch towards Sexson. Sexson later stated he was frustrated and had a lot going on in his head, including his club's recent struggles. Sexson was suspended six games for the incident,[4] but he successfully appealed it and the suspension was dropped to five games.

Gabbard has the standard repertoire of fastball, curveball, and changeup, occasionally throwing a slider. Gabbard's fastball hits the high-80s, his changeup works the best low and away on right-handed hitters, and his slow curveball is especially effective against lefties. Not a power strikeout pitcher, Gabbard has been compared to Mark Buehrle and Kenny Rogers.